The present invention relates to a method of imparting improved durability and abrasion resistance nano-structured optical surfaces, and more particularly to those formed from or on polymeric substrates or other optical elements or articles.
Most polymeric materials when used in a “mechanically harsh” environment (i.e. abrasion, or recurring impact) quickly wear away their usefulness. A method of coating these articles with “hardcoats” to impart robustness has been shown (e.g. hardcoated ophthalmic lenses, polymeric windows and laminates) to greatly improve useful life of the polymeric article (among others, U.S. Pat. Nos. 3,700,487; 4,049,868; 4,137,365; 4,611,892; 5,619,288 and 5,958,514.) In most applications were the polymeric material has some optical function, an anti-reflection (AR) coating is also required. It relies on a nano structure to create a Graded Refractive Index (GRIN) reduction to minimize reflection. This type of structure is well known and relies on the moths' eye principle (Bernard, C. G., Endeavor 26, pp. 79-84 (1967)).
However, the ideal or optimum GRIN structure is inherently mechanically unstable (i.e. foam like structure), and can be easily worn away with mild abrasion.
Numerous patents have been granted (for example see U.S. Pat. Nos. 2,445,238; 2,432,484; 3,490,982; 4,816,333; 4,374,158; 4,013,465; 4,114,983; 4,753,516 and 6,040,053) which have demonstrated that imparting a nanostructure to a surface dramatically reduced back reflection of visible light from it. However, none of these anti-reflective techniques produced a durable coating (Cathro et al. in “Silica Low-Reflection Coatings for Collector Covers by a Dye-Coating Process,” Solar Energy, Vol. 32, No. 5, pp. 573-579 (1984); and by J. D. Masso in “Evaluation of Scratch Resistant and Anti-reflective Coatings for Plastic Lenses,” Proceedings of the 32nd Annual Technical Conference of the Society of Vacuum Coaters, Vol. 32 p. 237-240 (1989)).
Lange et al. U.S. Pat. No. 4,816,333 discloses anti-reflective coatings of silica particles. The coating solution contains colloidal silica particles and optionally a surfactant (“Trition.TX.X-100” and “Tergitol TMN-6”) to improve the wettability of the coating solution. Tanigucki et al. U.S. Pat. No. 4,374,158 discloses an anti-reflective coating using a gas phase treatment technique. However, neither discloses the resultant abrasion resistance of their treatments. In fact, Taniguchi et al. state that a protective overcoat may be preferable with their nano-structure. They assert care should be taken not to make this layer too thin to negate the abrasion resistance, nor be made too thick to negate the AR effect.
It is therefore, an object of this invention to impart abrasion durability to a nano-structured surface. Among others, this invention can furnish mechanical durability (such as scratch, wear, and impact resistance) to an optical element that has a Moths' Eye type antireflection treatment on its surface.
It is another object of the invention to provide a process that can impart low surface energy properties to an optical element without difficulty and complication of process.
It is another object of the present invention to provide a method of imparting lubricity to an article that uses a nano-structure for its functionality (e.g. microdisplays, sensors, LEDs, memory devices, and other nano-technology devices).